Introduction
Okay, real talk: this whole chapter is one idea wearing different outfits. Everything is made of tiny particles that are always moving. Whether something is a solid, a liquid or a gas just depends on how those particles are arranged and how much they jiggle about. Lock that in and melting, boiling, gas pressure and diffusion all click into place.
Here's the cheat code the examiner secretly wants every single time: explain it with the particles. Not "it gets bigger", but "the particles move faster and spread out, so it gets bigger". This is your fast refresh, not the full textbook: one key idea per section, one quick worked example, and the exact move that scores the mark. Let's get into it. 💪
The Three StatesC1.1
The same particles can make a solid, a liquid or a gas. Only the arrangement and the movement change. Solid: neat rows, touching, only vibrating on the spot. Liquid: jumbled up, still touching, sliding past each other. Gas: far apart, flying around in all directions. That's it. Memorise the movement words and you're basically there: solids vibrate, liquids slide, gases fly.
> 🔑 Key idea: Solid = regular + vibrating. Liquid = random + sliding. Gas = far apart + fast. Always give the arrangement AND the movement. 🎯
Worked Example: Describing the StatesC1.1
Problem Sodium chloride is a solid at room temperature. Describe the particles in a solid, then say how a liquid and a gas differ.
Step 1 Solid: particles touching in neat rows, only vibrating on the spot.
Step 2 Liquid: still touching but jumbled, sliding past each other. Gas: far apart, moving fast in all directions.
Two marks per state come in pairs: arrangement + movement. ✅
Solid, Liquid or Gas?C1.1
Every property comes straight from the particles. Only a solid keeps its shape (particles locked in place). Solids and liquids keep their volume (particles touching). Only a gas can be squashed easily, because it has big gaps to close up, and only a gas has a really low density (same particles spread over way more space). So if something flows but you can't squash it smaller, that's a liquid.
> 🔑 Key idea: Keeps shape → solid. Keeps volume → solid or liquid. Squashes easily → gas. Big gaps = squashable + low density. 📦
Worked Example: Which State Is It?C1.1
Problem A substance takes the shape of any container but always keeps the same volume. Which state is it?
Step 1 Takes the container's shape → not a solid (a solid keeps its own shape).
Step 2 Keeps the same volume → not a gas (a gas spreads to fill everything). Flows but fixed volume = liquid. 🎯
Changes of StateC1.1
A change of state is just a substance switching between solid, liquid and gas. No new substance, totally reversible. Learn them as pairs: melting (solid→liquid) and freezing (liquid→solid); boiling/evaporating (liquid→gas) and condensing (gas→liquid). Going "up" takes in energy; going "down" gives it out. Bonus one: subliming jumps solid straight to gas (like dry ice). The trick is always checking which direction the change goes before you name it.
> 🔑 Key idea: Up (melt, boil) = takes in energy. Down (freeze, condense) = gives out energy. Check the direction before naming. 🔄
Worked Example: Naming ChangesC1.1
Problem For copper, change G turns liquid into gas, and change H turns liquid into solid. Name G and H.
Step 1 G is liquid → gas = evaporation (boiling).
Step 2 H is liquid → solid = freezing. (Don't mix it with melting, which is the opposite way.) ✅
Predicting the StateC1.1
Every pure substance has a fixed melting point and boiling point, and they chop the temperature scale into three zones. Below the melting point → solid. Between the two → liquid. Above the boiling point → gas. Just drop the given temperature onto that number line and read off the zone. Watch out for negative numbers, and always say the reason, not just the answer.
> 🔑 Key idea: Below melting = solid, between = liquid, above boiling = gas. Quote the comparison for the mark. 🌡️
Worked Example: The State of TinC1.1
Problem Tin melts at 232 °C and boils at 2602 °C. What is its state at 175 °C?
Step 1 Compare 175 °C with the melting point: 175 is below 232 °C.
Step 2 Below the melting point = it hasn't melted yet = solid. Reason: "175 °C is below the melting point of 232 °C". ✅
Heating Curves & the Flat BitsC1.1
Heat a solid steadily and graph temperature against time and you get a staircase: slopes (warming up) and flat parts (changing state). The weird bit: on a flat part you keep heating but the temperature doesn't go up. Why? The energy is being used to break the forces holding the particles together so they can change state, not to make them move faster. Once the change is done, the temperature climbs again.
> 🔑 Key idea: Flat part = changing state. Temperature stays still because the energy breaks the forces between particles, not speeds them up. ⏸️
Worked Example: What the Heat DoesC1.1
Problem While a solid melts you keep heating it, but the temperature doesn't rise. What is the heat used for?
Step 1 To melt, the particles must break free from their fixed positions.
Step 2 That needs energy to overcome the forces holding them in place, so the heat goes into breaking those forces, not into speeding up the particles. Temperature stays constant till it's all melted. 🔓
Evaporation vs BoilingC1.1
These both turn a liquid into a gas, so people mix them up, but they're different. Evaporation happens only at the surface and at any temperature below boiling (that's why a puddle dries on a cool day). Boiling happens all through the liquid and only at the boiling point (bubbles inside the liquid). For the mark, give both the place and the temperature.
> 🔑 Key idea: Evaporation = surface, any temp below boiling. Boiling = all through, only at the boiling point. State both. 💨
Worked Example: Spot the DifferenceC1.1
Problem A student says evaporation and boiling are the same because both make a gas. Give a correct difference.
Step 1 Evaporation: only at the surface, at any temperature below boiling.
Step 2 Boiling: all through the liquid, only at the boiling point. Two features each = full marks. ✅
Gas Volume: Heat & SqueezeC1.1
Gases change shape and size super easily because their particles are far apart and free. Two rules: heat a gas → bigger volume (faster particles spread out, at constant pressure); press harder on a gas → smaller volume (particles pushed closer, at constant temperature). The classic mix-up is the direction, so remember: temperature and volume go the same way, pressure and volume go opposite ways.
> 🔑 Key idea: Heat = bigger volume. More pressure = smaller volume. Temp & volume same way; pressure & volume opposite. 🎈
Worked Example: Two ChangesC1.1
Problem A fixed mass of gas is heated at constant pressure, then (separately) the pressure is increased at constant temperature. What happens to the volume each time?
Step 1 Heated at constant pressure → faster particles spread out → volume increases.
Step 2 More pressure at constant temperature → particles squeezed closer → volume decreases. ✅
Gas PressureC1.1
A gas pushes on its container because the particles keep hitting the walls — billions of tiny pushes add up to pressure. If the container is rigid (can't change size) and you heat it, the volume can't grow, so the pressure goes up instead: the faster particles hit the walls harder and more often. (A balloon can expand, but a steel can can't, so its pressure rises.)
> 🔑 Key idea: Pressure = particles hitting the walls. Rigid + heated → faster particles → hit harder & more often → pressure rises. 🥊
Worked Example: Heating a Sealed CanC1.1
Problem A rigid container of gas is sealed and heated. Which happen? 1: volume decreases. 2: molecules move faster. 3: pressure increases.
Step 1 Rigid container → volume can't change → statement 1 is wrong.
Step 2 Heating → molecules move faster (2 ✓) → they hit the walls harder and more often → pressure rises (3 ✓). Answer: 2 and 3. 🔥
DiffusionC1.2
Diffusion is particles spreading out by themselves, from where there are lots of them to where there are fewer, just because they're always moving randomly. No stirring needed. It's fastest in gases (particles fast + lots of space), slower in liquids, and basically zero in solids. Bonus: warming things up makes diffusion faster, because the particles move faster.
> 🔑 Key idea: Diffusion = spreading by random motion, high → low concentration. Faster in gases than liquids, and faster when warm. 🌫️
Worked Example: The Smelly RoomC1.2
Problem Perfume is opened at the front of a still room; soon someone at the back smells it. What process carried it, and how?
Step 1 The perfume particles are always moving randomly among the air particles.
Step 2 They spread out from the crowded bottle to the empty back of the room — that's diffusion. "Still room" means it's the random motion doing it, not a draught. 👃
Key Points to Remember
- Matter = tiny particles always moving. Solid: regular + vibrating. Liquid: random + sliding. Gas: far apart + fast.
- Only solids keep their shape; solids and liquids keep their volume; only gases squash easily and have low density.
- Changes of state: melting, freezing, boiling/evaporating, condensing, subliming. Up takes in energy; down gives it out.
- Find the state from melting/boiling points: below melting = solid, between = liquid, above boiling = gas.
- Heating/cooling curves have flat parts at the changes of state; the temperature holds because energy breaks the forces between particles.
- Evaporation = surface, any temperature below boiling. Boiling = all through the liquid, only at the boiling point.
- Heating a gas increases its volume (or its pressure if the volume is fixed); more pressure decreases the volume. Pressure = particles hitting the walls.
- Diffusion = spreading by random motion from high to low concentration; faster in gases than liquids, and faster when warm.